Nuclear defence and nuclear deterrence is an interesting concept. I'd like to argue that there's an element to it that is commonly missed.
The common view, one I had till not long ago, is that we only have nuclear weapons so that we can threaten to use it against anyone who threatens us. The idea being that threatening the total destruction that nuclear weapons imply will force others to do what we want or at least to not destroy us in return. This is of course a huge part of the strategic point of such weapons, but there is another element that I think is important.
There was a concept before 1945 of total war. In wars of great seriousness the only acceptable solution was the total destruction of one side or the other. In the American Civil war it was made clear that no accommodation or peace treaty would be brokered with the South, the aim was to force total and unconditional surrender. In the Second World War both sides bombed civilians and sank the ships carrying their food, the aim was to utterly destroy the other nation as a military power by terrorising its population. I want to argue that this has changed.
Now, wars are very much different now for a lot of different reasons, cultural and economic none the least. To borrow a line from Robert Wright "Among the many reasons that I think that we should not bomb the Japanese is that they built my mini-van." There are many reasons why total war now seems undesirable, but mostly these apply only to rich nations, or nations we really care about. Sure, we're not going to go to war with Japan, we buy all their stuff, but can the same reasoning really explain why we wouldn't wage total war against Yemen? Is there anyone with real power in the developed world who cares about Yemen? If Britain were to wage unrestricted to-the-death war with a small country like Yemen, the only thing it costs us is the army, and the cost of the army didn't stop us in Iraq or Afghanistan.
The change I would like to suggest is that with nuclear weapons every leader of a nuclear power has to make a choice before any armed conflict: Do I want to physically wipe this nation off the map. It is now literally within the power of the heads of 8 countries to stop their enemy existing. And yet they choose not to, because of MAD, because of nuclear fall out and for many other reasons, not greatest of which is morality and not least of which is fear. It is defiantly and permanently the case that no nuclear weapon will ever by used by one state on another. It cannot happen for all the self-interested and noble reasons that the world has come up with for the last half century. This changes things.
If we had gone into Iraq or Argentina or anywhere else and sought to genuinely wipe the other side out we could have. We chose not to and this changes the dynamic. No longer can hot headed generals shout “we must win at all costs”, no, not true, if we wanted to win at all costs we would have gone for total war. No longer can we say that the priority is to smash the enemy, it isn't. In modern wars we chose to only attack the government, or the rebels, or the violent faction, whichever, but we go into the conflict with the rule in mind that civilians are off limits. Much more than international law we are bound by consistency. If you wanted to take the fight to the Argentines then you should have gone nuclear, if you wanted to simply wipe out the Vietnamese you could have done it easily. There's still room to attack civilians, as Vietnam made clear, but that is never now the aim. Now when civilians are put in harm's way it is a side effect, when it happens there's a clear military argument against it rather than just a humanitarian one.
A world with nuclear weapons is one where fights are small and partial. Wars now must have sophisticated aims, generally to change the government from one faction to another. So tactics are much more refined and much less bloody. There's no justification for macho posturing, and so hopefully far fewer casualties. We should, I would tend to suggest, keep the ability to destroy our opponents, if only to prove to ourselves we dont want to.